Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 62

3.3.1.3 Self-Assembled Monolayer on Metal Oxide Surface
Although reasonable efficiencies have been reached with n-type metal oxides as the
electron selective layer in inverted solar, it still has great room for improvement,
as the surface of metal oxides have hydroxyl groups that can cause charge trapping at
the metal oxide/active layer interface [ 104 ]. These hydroxyl groups terminated
surfaces lead to high-interface charge recombination due to poor charge transfer.
One approach that can improve the electrical and morphological properties of metal
oxide/active layer interface is to utilize a SAM between the inorganic and organic
interface [ 40 , 91 , 105 - 109 ]. SAMs can be utilized to significantly modify the
interfaces of oxide and metallic surfaces to improve adhesion, compatibility, charge-
transfer properties, energy level alignment, and affect the upper layer growth of
materials. It was demonstrated that modifying the metal oxide surfaces of TiO 2 and
ZnO-based inverted solar cells with a fullerene-based SAM (C 60 -SAM) can improve
the device performance. The C 60 -SAM affects the photo-induced charge transfer at
the interface to reduce the recombination of charges, passivate inorganic surface trap
states, improve the exciton dissociation efficiency at the polymer/metal oxide
interface as well as act as a template to influence the overlayer BHJ distribution of
phases and crystallinity leading to higher efficiency inverted solar cells [ 105 , 110 ].
3.3.1.4 Polymer and Cross-Linked Interlayer on Metal Oxide Surface
The potential drawbacks for SAM formation on metal oxide surface are incomplete
coverage at the molecular scale and probable desorption of this monolayer during
wet processing, creating localized defects in this interlayer [ 109 ]. The other
approach that can improve the metal oxide/active layer interface is to insert an
organic ETL interlayer in between. In order to resist the solvent washing from the
over-layer, this organic layer should have orthogonal solubility with active layer, or
it should be cross-linkable. Choi et al. reported a remarkable improvement in
inverted solar cell performance by employing a thin layer of CPE on top of TiO x .
The TiO x /CPE composite ETL improves the electron injection and transport at the
cathode and blocks the hole transport to the cathode, leading to a PCE improvement
from 2.65 to 3.55 % [ 111 ]. The CPE material was alcohol soluble, and thus not
affected by the solvent of the active layer. Hsieh et al. reported a PC 61 BM-based
n-type material [6,6]-phenyl-C 61 -butyric styryl dendron ester (PCBSD) function-
alized with a dendron containing two styryl groups as thermal cross-linkers [ 112 ].
By heating at 160 C for 30 min a robust, adhesive, and solvent-resistant thin film
can be generated on top of ZnO layer. An inverted solar cell device based on ITO/
ZnO/cross-linked PCBSD/P3HT:PC 61 BM/PEDOT:PSS/Ag configuration not only
achieves enhanced device characteristics (PCE 4.4 %), but also exhibits an
exceptional device lifetime without encapsulation; it greatly outperforms a refer-
ence device (PCE 3.5 %) based on an ITO/ZnO/P3HT:PC 61 BM/PEDOT:PSS/Ag
configuration without the interlayer. Changing the acceptor in the active layer from
PC 61 BM to a novel fullerene derivative indene-C 60 bis-adduct (ICBA) can further